Limit the number of decimal and length of number with regex in c# -

i have been trying validate number limits decimal point 3 points , length of number (including dot) 6, couldn't validate properly. far have:

^([0-9]([.][0-9]{1,3})?){1,6}$ 

how change regex above validate properly? thanks.

you can use lookahead based regex checking length:

^(?=[0-9.]{1,6}$)[0-9]+(?:\.[0-9]{1,3})?$ 

regex demo